1. Field of the Invention
The present invention relates to an imaging system, and in particular an X-Ray imaging system, which provides a frame subtraction feature in order to enhance images produced by such a system.
2. Description of the Related Art
Imaging systems are used to allow users to analyze objects using optical, or other scanning means which may be converted to an image display. In some situations, the difference between a first image frame and one or more other image frames is of interest to the user.
For example, a medical imaging system, and in particular an X-Ray imaging system, is often used to analyze blood vessels within a patient. In such a situation, a dye which is substantially opaque to X-Rays is injected into the patient to be pumped to the desired blood vessels within the body. A sequence of X-Ray frames is taken of the desired area of the body to detect the entry and path of the dye within the blood vessels, and thus to display for the user an image of the size and location and operation of the blood vessels in that area. However, image artifacts produced by other parts of the body in the area being observed can obscure or otherwise distract the user from the primary goal of analyzing the blood vessels. Referring to FIG. 1, the center image 104 is an example of such an image.
Systems exist which will select an image frame of the desired area during the time before the dye is detected. Such an image frame is termed a mask frame in the present application and represents a background or quiescent image of the area of interest. Referring again to FIG. 1, the right hand image 106 represents a mask frame. This mask frame is saved, and subtracted from the respective subsequent sequence of image frames, sometimes termed fill frames, showing the dye entering and leaving the desired area. By subtracting a ‘non-dye’ image from the subsequent ‘dye’ image, the image of the other body parts is minimized, leaving only the image of the dye in the blood vessel. Referring again to FIG. 1, the left hand image 102 represents this ‘difference’ image. In image 102, artifacts representing the surrounding anatomical features are minimized, and the blood vessels, highlighted by the dye, are more easily seen and analyzed. This is termed a frame subtraction feature in the present application.
More generally, in such imaging systems, a mask frame may be designated and stored at a time before or after a particular test is performed, although typically the mask frame is taken at a time immediately before the sequence of image frames showing the test results. The mask frame is subtracted from the respective sequence of frames which are taken during the test. This will generate a sequence of frames in which the differences between the area of interest during a test and the area of interest before (or after) the test are highlighted, allowing a user to more easily see the results of the test.
However, it is also sometimes desirable to move the X-ray source and detector (typically rigidly attached via a C-arm) with respect to the patient, or to move the table on which the patient is laying, during the course of the test, e.g. to follow an observed anomaly or improve the view of the desired area. If the C-arm and/or table is moved while maintaining the same relative spacing of the patient between the X-ray source and the detector, then the respective locations of the subsequent sequence of frames taken during a test is offset from the location at which the mask frame was taken. This may be compensated for by shifting the mask frame so that the mask frame remains aligned with the respective sequence of subsequent images.
For example, during the test, the C-arm and/or the table on which the patient is lying may be moved. The imaging system maintains data representing position of the table and the position of the C-arm. The table position and C-arm position data at the time of each image frame is stored with the image data representing that image frame. During display of the X-ray image frames, the orientation and/or position data is retrieved for the respective subsequent image frames and the mask frame shifted the appropriate amount to maintain the alignment with the subsequent sequence of frames.
However, it may be further desirable to change the position of the patient closer to or farther away from the imaging system detector. This results in a sequence of images which is relatively enlarged or reduced, i.e. zoomed. In this situation, the mask frame which was previously taken and saved no longer represents an accurate representation of the background or quiescent image as presented in the subsequent sequence of images. Subtracting such a mask frame from the subsequent sequence of images will not result in highlighting the test results by minimizing the background or quiescent image, and may even result in obscuring the test results.
FIG. 2 is an X-ray image illustrating the difference between a mask frame and a fill image frame which has been zoomed with respect to the mask frame. In FIG. 2, a mask frame is taken and stored with the subject at a first distance between the X-ray source and detector, and a fill frame is taken with the same subject at a second distance between the X-ray source and detector. More specifically, the image of FIG. 2, shows a motor 202 and a small round disk 204. The motor 202m appears light and the disk 204m appears white in the mask frame, and the motor 202f and the disk 204f appear black in the subsequent fill frame. As may be seen, the image of the motor 202m and disk 204m appear smaller in the mask frame than in the fill frame, 202f, 204f. In addition, the images of the motor 202m and disk 204m in the mask frame are displaced from the respective images of the motor 202f and disk 204f in the fill frame. This implies that the mask frame was taken and stored while the subjects, e.g. motor 202 and disk 204, were further from the detector than when the fill frame was taken. It may be seen that the mask frame does not correspond in size with the fill frame and, therefore, is misaligned with non-mask image. Consequently, the resulting subtraction frame illustrated in FIG. 2 does not accurately eliminate background and/or quiescent artifacts. It may also be seen that there is no amount of frame shifting (described above) which will bring the mask frame image and fill frame image into alignment.
An imaging system is desirable which will permit a frame subtraction feature to be used when a patient is moved closer to or further from the detector in the imaging system in the manner described above.